1. Field of the Invention
The present invention concerns a method and a device for phase-sensitive flow measurement by means of a volume segment of an examination subject for a magnetic resonance system.
2. Description of the Prior Art
In phase-sensitive flow measurements or phase contrast flow measurements conducted with a magnetic resonance system, bipolar gradients are switched (activated) to encode a flow for measurement of the flow of tissue fluids (for example blood). These bipolar gradients lead to eddy current effects. These eddy current effects lead to background phase that in MR images containing velocity information, which are also called phase images, superimposes on the actual velocity information and therefore causes an adulterated flow velocity depiction. In other words, the background phase adulterates the quantitative information that is to be evaluated to measure flow and velocity values. An additional source of error is the gradient non-linearity. In addition to errors in the spatial association of the measured signal, or the measured MR data, this gradient non-linearity leads to errors in the flow coding, which likewise negatively affects the measurement of flow and velocity values. These two effects (eddy current effects, gradient non-linearity) increase with the distance from the isocenter of the magnetic resonance system and can already cause significant error contributions at distances of a few centimeters from the isocenter.
According to the prior art it is sought to avoid these two effects in flow measurements by positioning the image regions of interest (vessels, for example) as close as possible to the isocenter. With regard to the z-direction, given a predetermined position of the examination subject on the table of the magnetic resonance system it is possible by displacement of the patient table or by another technique, to place the image region to be acquired optimally close to the isocenter. This approach according to the prior art is able to keep the consequences of the two effects small only if image regions are measured with a correspondingly small extent in the z-direction. However, if the region to be measured (i.e, the region from which image data are to be acquired) encompasses an image region with a larger extent in the z-direction (for example more than 10 cm), the consequences of the two effects can be kept small only for a portion of the region that lies in the center of the image region to be measured, while the background phase effects are unavoidable in other regions that lie far removed from the isocenter.
Due to the increasing importance of flow measurements with volumetric coverage, for example “4D Flow” (generation of velocity information over time in a three-dimensional space), the correction or avoidance of the errors occurring due to the background phase effects becomes ever more important. According to the prior art image-based corrections are known via which the errors in the background phase and errors also generated by the gradient non-linearity can be corrected, but only partially.